fix(arborist): repair wrong-but-existing symlink target in linked strategy

[manzoorwanijk]

In continuation of our exploration of using install-strategy=linked in the Gutenberg monorepo, which powers the WordPress Block Editor.

Under install-strategy=linked, if a top-level node_modules/<dep> symlink points to a store key that exists on disk but is the wrong version, re-running npm install does not repair it. npm reports success and leaves the dependency resolving to the wrong version. This is the state an interrupted update leaves behind: the new store key is extracted but the symlink has not yet been repointed.

A symlink pointing at a non-existent target is already repaired on reinstall; only a wrong-but-existing target slips through, because cleanup validates the link name, not its target.

Why

For linked installs, #buildLinkedActualForDiff synthesizes the "actual" tree the diff compares against from the ideal children, never reading the real on-disk symlink target. So a link whose on-disk target is a valid-but-wrong store key looks identical to the ideal node, the diff reports no change, and the symlink is left untouched. The hoisted strategy is unaffected because it self-heals the analogous corruption.

How

In #buildLinkedActualForDiff, when an existing link's resolved on-disk target differs from its ideal target, skip creating a synthetic actual entry for it. With no actual entry to match, the diff treats the link as an ADD, and #reifyNode removes the old symlink and recreates it pointing at the correct store key. A new #linkTargetMismatch helper compares the two resolved targets; it runs only after the existing existsSync guards, so both paths are known to exist.

This repairs both the top-level symlink and wrong transitive/sibling links inside the store, and leaves already-correct trees untouched (no spurious relinking on an idempotent reinstall).

References

Fixes #9611

[github-actions]

🎉 Backport to release/v11 created: #9644